There are quite few other advanced techniques that exist. None of BrainBashers Sudoku need them as we don't believe that a human can be reasonably expected to use them, although some are technically very simple.

They may require you to spot numbers that are linked together, or some iterative technique, or perhaps trial and error. The following is a non-exhaustive list of techniques, and Google would be a good place to start if you would like more information.

Loops

Chains

Colourings

Nishio

Fishy Cycles

Bowman Bingo

Trebor's Tables

and many more...

If you ever come across a Sudoku that requires any of these techniques, without being warned, then you've been given a task we wouldn't want!